Medical electrodes have in the past taken many shapes and forms. Principally, they have been shaped according to the use for which they are intended. Electrodes used in monitoring apparatus, such as EKG and EEG machines, commonly have small round contact surfaces. While electrodes used in such stimulation apparatus as pain control tend to be larger and have most often rectangularly or other conveniently shaped contact surfaces. Whether intended for monitoring or stimulation use, a design objective for each electrode group has been, and continues to be, good electrical signal transmission between a patient'skin surface and the electrical wiring connected to a particular piece of apparatus. Not only is efficient signal transmission across the epidurumconductor interface desirable, but so is effective signal transmission which is free of current concentration points or "hot spots."
Prior art electrodes offer combination structures including a metallic or otherwise conductive support member to which an electrical wire from an associated apparatus may be attached. Some electrodes teach the incorporation of an electrode paste or gel applied directly to the conductive support member to enhance conductivity across the skin-electrode interface.
Other electrodes teach the additional incorporation of an open cellular skin interface pad secured to a conductive support member. This pad as shown in U.S. Pat. No. 3,817,252, is very often a sponge material and functions to hold or contain an electrolyte solution. The electrolyte solution enhances conductivity across the skin-pad interface. Alternately, this interface pad can be saturated with electrode pastes or gels which will not turn or evaporate as readily as electrolyte solutions.
None of these prior art electrodes offer a structure which will maintain constant, efficient and effective electrical transmission for long periods of time without the need for additional electrode paste, gel or solution. Moreover, with these electrodes there is a tendency for the electrolyte film to separate and/or to flow to a non-uniform thickness. Under these conditions, sections of the conductive support member could be exposed to the skin. Local hot spots will result which can cause discomfort to the patient if not causing burns to the patient's skin.
These prior art electrodes must be secured to the surface of a patient's skin with medical tape or other securing mediums. Very often an electrode secured in this manner will pull away from skin creating a partial or total interruption in signal transmission.
More recent improvements in the electrode art include composite electrodes using electroconductive tape as the skin interfacing medium. This tape has a film of pressure sensitive adhesive for engaging the skin surface. In these electrodes, the adhesive has been doped with a quantity of electrically conductive particles such as carbon powder as disclosed in U.S. Pat. No. 3,911,906 in order to provide an electrical path to the skin. This doping can create non-uniform electrical transmission through the adhesive. Moreover, in the presence of large quantitites of skin moisture, these adhesives lose their ability to adhere to the skin surface, thus pulling away and drastically changing the electrical characteristics of the electrode.
An objective of this invention, therefore, is to provide a composite electrode with a skin-interface substrate which will perform a similar function to, and therefore eliminate the need for an electrolyte solution, electrode paste or electrode gel.
Another objective of this invention is to provide this electrode with a skin-interface substrate having adhesive properties which will enable the electrode to adhere to the skin without the use of tape or other securing means and which will not lose adhesiveness in the presence of large quantitites of skin moisture.
Another objective is to provide this electrode wherein the adhesive skin-interface substrate is electrically conductive this conductivity being uniform throughout the substrate.
A further objective is to provide this electrode with a skin-interface substrate which will maintain a uniform thickness and will not separate to expose sections of a conductive support member to the skin.
An even further objective is to provide such an electrode including such a skin-interface substrate where this substrate will not break down under long periods of use.